Conventionally, quenching and tempering is used to obtain high-strength structural steels with good impact toughness and elongation. However, tempering is additional process step requiring time and energy because of re-heating from temperatures below Mf after quenching.
In recent years, sophisticated high strength steels with improved toughness are achieved advantageously by direct quenching. However, the ductility of these steels in terms of their elongation or reduction of area to fracture in uniaxial tensile testing is generally acceptable, but their uniform elongation, i.e. work hardening capacity could be improved. This deficiency is an important factor limiting the wider and more demanding application of such steels because strain localization during fabrication or as a result of overloading in the final application can be detrimental to the integrity of the structure.
Due to an ever-increasing demand for advanced high-strength steels (AHSS) with excellent toughness and reasonable ductility and weldability, fresh efforts have been directed to develop new compositions and/or processes to meet the challenges of the industry. Within this category, the dual-phase (DP), complex phase (CP), transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) steels have been developed during the past few decades, mainly to meet the requirements of the automotive industry. The main aims have been to save energy and raw materials, improve safety standards and protect the environment. So far, the yield strength of the above AHSS steels with carbon content in the range of 0.05 to 0.2 wt. % has been usually restricted to about 500 to 1000 MPa.
Patent publication US2006/0011274 A1 discloses a relatively new process called quenching and partitioning (Q&P) which enables the production of steels with microstructures containing retained austenite. This known quenching and partitioning process consists of a two-step heat treatment. After reheating in order to obtain either a partially or fully austenitic microstructure, the steel is quenched to a suitable predetermined temperature between the martensite start (Ms) and finish (Mf) temperatures. The desired microstructure at this quench temperature (QT) consists of ferrite, martensite and untransformed austenite or martensite and untransformed austenite. In a second partitioning treatment step, the steel is either held at the QT or brought to a higher temperature, the so-called partitioning temperature (PT), i.e., PT≧QT. The aim of the later step is to enrich the untransformed austenite with carbon through depletion of the carbon-supersaturated martensite. In the Q&P process, formation of iron carbides or bainite is intentionally suppressed, and the retained austenite is stabilized to get the advantage of strain-induced transformation during subsequent forming operations.
The above developments were intended to improve the mechanical and forming related properties of thin sheet steels to be used in automotive applications. In such applications, good impact toughness is not required and yield strengths are limited to below 1000 MPa.
The target of this invention is to accomplish, preferably without additional heating from temperatures below Mf after quenching, a structural steel product having a yield strength Rp0.2 of at least 960 MPa and excellent impact toughness, such as 27J Charpy V transition temperature ≦−50° C., preferably ≦−80° C. together with good total uniform elongation.
However, even though the best practice is to utilize the invention within the field of structural steels, it should be understood, that the referred method and steel product according to the invention can also be used as a method for manufacturing hot-rolled wear resistant steels and that the referred high-strength structural steel product can be used as hot-rolled wear resistant steels, even though such good impact toughness and ductility is not always required in wear resistant steel applications.